Preheat your oven to 350°F (175°C).
In a bowl, whisk together flour, cocoa powder, baking soda, and salt.
In another bowl, cream the softened butter and brown sugar until light and fluffy.
Add the egg and vanilla extract to the butter mixture, mixing well.
Gradually incorporate the dry ingredients into the wet mixture.
Fold in the chocolate chips until evenly distributed.
Scoop tablespoon-sized balls of dough onto a baking sheet lined with parchment paper.
Top each dough ball with mini marshmallows.
Bake in the preheated oven for 10-12 minutes.
Let cookies cool for a few minutes, then drizzle melted chocolate on top.